1. Field of the Invention
The present invention relates generally to a semiconductor laser and a method for manufacturing the same.
2. Related Background Art
Semiconductor lasers for the wavelength of 780 nm using AlGaAs as the material of their active layers (light-emitting layers) have been brought into practical use as writing light sources of optical disks such as CD (compact disk). CDs using this type of semiconductor lasers include CD-R (Recordable) for single time writing and CD-RW (ReWritable) for plural time writing. These writing optical disks have been involved in so-called double-speed races for increasing the disk rotation speed to raise the access speed to several times the standard speed and to increase the magnification. For making one data bit with such a writing optical disk, it is necessary to give certain pulse energy. As the disk rotation speed increases, it is necessary to increase the optical power of the irradiated laser. Therefore, semiconductor lasers for high-speed writing are required to produce high outputs.
In the above-mentioned AlGaAs semiconductor lasers, a ridge-guided structure is often used because high characteristics can be obtained by easy manufacturing methods. Semiconductor lasers having the ridge-guided structure include those of a complex index-guided structure and those of a real index-guided structure. The latter real index-guided structure is less in absorption loss and is remarked as a high throughput semiconductor laser structure.
FIG. 5 is a cross-sectional schematic view showing an existing semiconductor laser with a real index-guided structure. On an n-type GaAs substrate 401, sequentially formed are an n-type clad layer 402 made of Al0.5Ga0.5As, guide layer 403 made of Al0.3Ga0.7As, AlGaAs/AlGaAs active layer 404, guide layer 405 made of Al0.3Ga0.7As and first p-type clad layer 406 made of Al0.5Ga0.5As. Formed on the first p-type clad layer 406 is formed a second p-type clad layer (ridge waveguide) 407 made of Al0.5Ga0.5As in a stripe shape. The ridge waveguide is usually formed by reaction rate controlling wet etching that exposes the (111) A plane. Therefore, as shown in FIG. 5, it becomes trapezoidal with the top width Wu narrower and the bottom width Wd wider. A current-blocking layer 408 made of n-type Al0.55Ga0.45As is formed to lie in opposite sides of the ridge waveguide 407. Then a contact layer 410 made of p-type GaAs overlies the current-blocking layer 408 and the ridge waveguide 407. In the semiconductor laser shown in FIG. 5, a current is injected into the active layer 404 from a p-side electrode 412 and an n-side electrode 411, and laser light L of a wavelength around 780 nm is emitted from the active layer 404. When a current is injected to the active layer 404, the above-mentioned current-blocking layer 408 functions to constrict the current to the area just under the ridge waveguide 407. Additionally, the current-blocking layer 408 produces a difference in refractive index between the portion under the ridge waveguide 407 and the portions at its opposite sides to thereby confine the laser light L in the lower portion of the ridge waveguide 407. In the semiconductor laser of FIG. 5, the refractive index of the current-blocking layer 408 is smaller than those of the p-type clad layers 406, 407. This type of structure relying on refractive indices is called a real index-guided structure. In this structure, the band gap of the current-blocking layer 408 is larger than that of the active layer 404, and the current-blocking layer 408 is translucent for the light L from the active layer 404. Therefore, its absorption loss is small and the throughput is relatively high.
If there were a writing laser device operable at a higher speed than the existing writing laser devices, it could be effectively used for various purposes including CD-R, etc. mentioned above. To realize such a high-speed writing semiconductor laser, a semiconductor laser with a still higher output than conventional writing semiconductor lasers is indispensable. However, the use of a conventional semiconductor laser as shown in FIG. 5 to produce a higher output than now has been considered to be very difficult because it is difficult to accomplish both narrowing the bottom width Wd of the ridge waveguide 407 to raise the kink level and increasing the thickness if the ridge waveguide to reduce the absorption loss by extrusion of light to the GaAs contact later 410.
That is, as shown in FIG. 6, the optical output Po of a semiconductor laser can be certainly enhanced by an increase of the operating current Iop. However, the rate of increase of the optical output relative to the operating current Iop suddenly changes when it reaches a certain value Pk. This is the phenomenon called a kink that derives from primary mode oscillation L1 (FIG. 5) being liable to occur in addition to basic mode oscillation L (FIG. 5) as the normal laser oscillation due to influences of hole burning, etc. That is, the transverse mode of the laser light becomes unstable at the kink level. However, laser light must be stable in semiconductor lasers because the laser light is converged to a minute spot when it is used. Therefore, in order to obtain a semiconductor laser usable for a high output, it is essential to raise the kink level Pk. To raise the kink level Pk in the semiconductor laser of FIG. 5, it is considered desirable to converge the current injected into the active layer 404 by minimizing the bottom width Wd of the ridge waveguide 407. On the other hand, from the viewpoint of reducing the absorption loss by extrusion of light to the contact layer 410 in the semiconductor laser of FIG. 5, it is rather desirable to increase the thickness of the ridge waveguide 407. However, if the bottom width Wd of the ridge waveguide 407 is narrowed while the thickness of the ridge waveguide 407 is maintained thick, the top width Wu excessively narrows and increases the threshold voltage so much to make laser oscillation difficult. As such, the conventional semiconductor laser shown in FIG. 5 is difficult to enhance the confinement of light in the active layer 404 and simultaneously increase the kink level. It is therefore difficult for the semiconductor laser to be raised in kink level than before and used for a higher output.